Wednesday, October 12, 2011

Deadly Java Quake Highlights "Ring of Fire" Dangers

The magnitude-6.3 earthquake that shook the Indonesian island of Java this weekend has so far killed about 5,700 people and left more than 100,000 homeless.

The devastating quake is at least the fourth geological disaster to strike Indonesia in 18 months (Indonesia map, music, and profile), highlighting the constant threat faced by residents of the Pacific Ocean zone known as the Ring of Fire. This tectonically active region rims the Pacific and includes Mount St. Helens, now erupting in Washington State.

The event also puts a spotlight on scientists' efforts to understand how and why quakes can cause different kinds and levels of damage.
On December 26, 2004, the Indonesian island of Sumatra was hit by the largest earthquake the world had seen in 40 years—a magnitude 9.3.
The massive quake triggered a tsunami that left about 131,000 dead in Indonesia alone (news reports: "Tsunami in Southeast Asia: Full Coverage").
That event was followed on March 28, 2005, by another enormous earthquake, with a magnitude of 8.7, sparking fears of a second tsunami. But the deadly wave didn't materialize, and the death toll was about 320.
More recently one of Indonesia's many volcanoes, Mount Merapi, began erupting only a few dozen miles from the epicenter of this weekend's quake (related photo: Volcano Eruption Countered With Dance, Food).
The latest quake occurred on Saturday at 5:54 a.m. local time and had a magnitude of 6.3, the U.S. Geological Survey (USGS) reported on its Web site.

Same Plates, Different Results

Earthquakes and volcanoes are produced by the forces of plate tectonics, which cause the vast plates that form the Earth's crust to slowly but steadily collide.
Just south of Java, for example, the Australian plate is moving northward at about two and a half inches (six centimeters) a year.
When the Australian plate collides with the Sunda plate, which includes Java, the Australian plate is forced beneath the Sunda plate in a process called subduction.

Earthquakes result when the subducting crust gets stuck, then lurches back into motion. Volcanoes are formed when subducted rock melts and returns to the surface as magma. (Learn more about how and why earthquakes happen.) The large Indonesia earthquakes of December 2004 and March 2005 were caused by a similar plate collision off the island of Sumatra.

But the Java earthquake has some important differences, Mark Leonard, senior seismologist with the government organization Geoscience Australia in Canberra, wrote in an email.
Not only was the epicenter of the Java quake several hundred miles away from Sumatra, but the motion was of a type geologists refer to as strike-slip, Leonard says.
Strike-slip earthquakes involve sideways motion along a fault.
The 2004 and 2005 earthquakes were "thrust" earthquakes, in which the two sides of the fault were rammed more directly toward each other.
Also, Leonard says, this weekend's earthquake originated only 18.6 miles (30 kilometers) beneath the surface.

Usually subduction-zone earthquakes in this region occur 40 to 60 miles (70 to 100 kilometers) deep.
"This all suggests that this earthquake was possibly not on the main subduction zone," he wrote, "but on a shallower [unmapped] strike-slip fault [in the overlying crust]."
"I am speculating," he added, "but if this is the case, [the shallow nature of the quake] would explain why the damage is greater than for other magnitude 6.0 to 6.5 earthquakes in the last couple of decades."

Shaken Temples

The damage has been significant, with many villages destroyed and two of Indonesia's major cultural sites affected.
Borobudur, the largest Buddhist temple on Earth (see photo), was built 1,200 years ago about 25 miles (40 kilometers) north of the royal capital of Yogyakarta (pronounced JOG-jakarta), the major city in the affected region.

The temple lay abandoned for centuries before being rediscovered and restored in the early 1900s. The recent quake damaged the structure, but it appears to be mostly intact.
Prambanan, the oldest Hindu temple in Indonesia, was not so fortunate. This complex, built about 1,150 years ago, appears to have taken significant damage, according to newspaper reports.
The epicenter for Saturday's earthquake is also only a few dozen miles from the erupting Merapi volcano. But this doesn't mean that the eruption triggered the quake.
In a statement posted to its Web site, the U.S. Geological Survey notes that magma movements at volcanoes can produce shallow earthquakes.

"In the cases of many earthquakes that occur in the general vicinity of volcanoes, however, there are not obvious links to volcanic eruptions," the agency said.
Leonard, of Geoscience Australia, was more certain.
Though Merapi and the recent earthquake were produced by the same large-scale tectonic forces—the slow collision of the Sunda and Australian plates.
The fact that the quake occurred during the eruption "is a coincidence," he wrote.